Metal complexes with boron-nitrogen heterocycle containing ligands

ABSTRACT

Novel organic compounds comprising ligands containing a boron-nitrogen heterocycle are provided. In particular, the compound is a metal complex comprising a ligand containing an azaborine. The compounds may be used in organic light emitting devices to provide devices having improved photophysical and electronic properties.

This application claims priority to U.S. Provisional Application Ser. No. 61/179,933, filed May 20, 2009, the disclosure of which is herein expressly incorporated by reference in its entirety.

The claimed invention was made by, on behalf of, and/or in connection with one or more of the following parties to a joint university corporation research agreement: Regents of the University of Michigan, Princeton University, The University of Southern California, and the Universal Display Corporation. The agreement was in effect on and before the date the claimed invention was made, and the claimed invention was made as a result of activities undertaken within the scope of the agreement.

FIELD OF THE INVENTION

The present invention relates to novel organic materials that may be advantageously used in organic light emitting devices (OLEDs). More particularly, the present invention relates to novel compounds comprising a boron-nitrogen heterocycle and devices containing such compounds.

BACKGROUND

Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants.

OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.

One application for phosphorescent emissive molecules is a full color display. Industry standards for such a display call for pixels adapted to emit particular colors, referred to as “saturated” colors. In particular, these standards call for saturated red, green, and blue pixels. Color may be measured using CIE coordinates, which are well known to the art.

One example of a green emissive molecule is tris(2-phenylpyridine) iridium, denoted Ir(ppy)₃, which has the structure:

In this, and later figures herein, we depict the dative bond from nitrogen to metal (here, Ir) as a straight line.

As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules.

As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between.

As used herein, “solution processible” means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form.

A ligand may be referred to as “photoactive” when it is believed that the ligand directly contributes to the photoactive properties of an emissive material. A ligand may be referred to as “ancillary” when it is believed that the ligand does not contribute to the photoactive properties of an emissive material, although an ancillary ligand may alter the properties of a photoactive ligand.

As used herein, and as would be generally understood by one skilled in the art, a first “Highest Occupied Molecular Orbital” (HOMO) or “Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. Since ionization potentials (IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative). Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative). On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. A “higher” HOMO or LUMO energy level appears closer to the top of such a diagram than a “lower” HOMO or LUMO energy level.

As used herein, and as would be generally understood by one skilled in the art, a first work function is “greater than” or “higher than” a second work function if the first work function has a higher absolute value. Because work functions are generally measured as negative numbers relative to vacuum level, this means that a “higher” work function is more negative. On a conventional energy level diagram, with the vacuum level at the top, a “higher” work function is illustrated as further away from the vacuum level in the downward direction. Thus, the definitions of HOMO and LUMO energy levels follow a different convention than work functions.

More details on OLEDs, and the definitions described above, can be found in U.S. Pat. No. 7,279,704, which is incorporated herein by reference in its entirety.

SUMMARY OF THE INVENTION

A novel class of organic materials are provided. The materials are compounds with boron-nitrogen heterocycle containing ligands. The materials may be advantageously used in OLEDs. In particular, the materials may be used as an emitting dopant in such devices.

Compounds comprising a boron-nitrogen heterocycle are provided, such compounds comprising a ligand L having the structure:

A and B may represent a 5-membered or 6-membered carbocycle or heterocycle. B is a boron-nitrogen heterocycle. X₁ is selected from the group consisting of carbon and nitrogen. Preferably, X₁ is nitrogen. Preferably, A is:

In particular, R₁ may be hydrogen.

In one aspect, A is selected from the group consisting of:

X is selected from the group consisting of S, NZ, O, Se, BZ, CZZ′, and C═O. Z and Z′ are independently selected from the group consisting of hydrogen, alkyl, and aryl.

In another aspect, A is selected from the group consisting of:

X is selected from the group consisting of S, NZ, O, Se, BZ, CZZ′, and C═O. Z and Z′ are independently selected from the group consisting of hydrogen, alkyl, and aryl.

X₂ and X₃ are independently selected from the group consisting of carbon, nitrogen, and boron.

R₁ and R₂ represent mono, di, tri, or tetra substitutions. R₁ and R₂ are independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl.

The ligand L is coordinated to a metal M having an atomic number greater than 40. Preferably, the metal M is Ir.

In one aspect, the compound provided comprise a ligand L having the structure:

X₄, X₅, X₆, and X₇ are independently selected from the group consisting of carbon, nitrogen, and boron.

In another aspect, the compound provided comprise a ligand L having the structure:

X₄, X₅, and X₆ are independently selected from the group consisting of carbon, nitrogen, and boron.

In yet another aspect, particular compound comprising a ligand L are provided wherein the ligand L is selected from the group consisting of Compound 1-Compound 67. R₃, R₄, R₅, and R₆ are independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl.

Preferably, the compound have a ligand L selected from the group consisting of Compound 1, Compound 6, Compound 7, Compound 12, Compound 25, and Compound 28. More preferably, the ligand is Compound 25.

R₂ may be hydrogen. R₃ may be selected from the group consisting of alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl.

In one aspect, compounds are provided wherein the ligand L is included in a homoleptic compound. In another aspect, compounds are provided wherein the ligand L is included in a heteroleptic compound.

In particular, compounds having the formula M^(n)(L)_(a)(L′)_(b)(L″)_(c) are provided. n is the oxidation state of the metal M. a is 1, 2, or 3. b is 0, 1, or 2. c is 0, 1, or 2. a+b+c is n. L′ and L″ are independently selected from the group consisting of:

R′, R″ and R′″ are independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl. X is selected from the group consisting of S, NZ, O, Se, BZ, CZZ′, and C═O. Z and Z′ are independently selected from the group consisting of hydrogen, alkyl, and aryl.

Preferably, L′ and L″ have FORMULA IV and are independently selected from the group consisting of Compound 68-Compound 83.

Particular boron-nitrogen heterocycle containing compounds are provided, wherein the compound is selected from the group consisting of Compound 84G-Compound 167G. Specific example of boron nitrogen heterocycle containing compounds are provided, including compounds selected from the group consisting of Compound 84-Compound 167. Preferably the compound is Compound 88 or Compound 89.

An organic light emitting device is also provided. The device may include an anode, a cathode, and an organic layer disposed between the anode and the cathode. The organic layer comprises a compound comprising a ligand L having the structure of FORMULA I, as described above.

A consumer product comprising a device is also provided. The device further comprises an anode, a cathode, and an organic layer disposed between the anode and the cathode. The organic layer comprises a compound comprising a ligand L having the formula of FORMULA I, as described above.

In one aspect, the organic layer is an emissive layer and the compound comprising the ligand L having FORMULA I is an emitting dopant. The organic layer may further comprise a host containing a benzene, carbazole, dibenzofuran, dibenzothiophene, dibenzoselenophene, naphthalene, triphenylene, or biphenyl moiety.

Selections for the rings, metal, and substituents described as preferred for compounds comprising a ligand L having FORMULA I are also preferred for use in a device that contains a compound comprising a ligand L having FORMULA I or a consumer product comprising a device that contains a compound comprising a ligand L having FORMULA I. These selections include those for rings A and B, the metal M, the substituents X₁-X₃, the substituents R₁-R₆, R′, R″, R′″, and the combination and/or incorporation of ligands L′ and L″ into an organometallic complex.

Additional compounds comprising a boron-nitrogen heterocycle are provided, such compounds having the formula:

A and B may represent a 5-membered or 6-membered carbocycle or heterocycle. X₁ is N or NR. X₂ and X₃ are independently selected from the group consisting of carbon, nitrogen, and boron. B is a boron-nitrogen heterocycle. R, R₁ and R₂ represent mono, di, tri, or tetra substitutions. R, R₁ and R₂ are independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device.

FIG. 2 shows an inverted organic light emitting device that does not have a separate electron transport layer.

FIG. 3 shows the general structure of a ligand containing a boron-nitrogen heterocycle.

FIG. 4 shows exemplary ligands containing a boron-nitrogen heterocycle.

DETAILED DESCRIPTION

Generally, an OLED comprises at least one organic layer disposed between and electrically connected to an anode and a cathode. When a current is applied, the anode injects holes and the cathode injects electrons into the organic layer(s). The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an “exciton,” which is a localized electron-hole pair having an excited energy state, is formed. Light is emitted when the exciton relaxes via a photoemissive mechanism. In some cases, the exciton may be localized on an excimer or an exciplex. Non-radiative mechanisms, such as thermal relaxation, may also occur, but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from their singlet states (“fluorescence”) as disclosed, for example, in U.S. Pat. No. 4,769,292, which is incorporated by reference in its entirety. Fluorescent emission generally occurs in a time frame of less than 10 nanoseconds.

More recently, OLEDs having emissive materials that emit light from triplet states (“phosphorescence”) have been demonstrated. Baldo et al., “Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices,” Nature, vol. 395, 151-154, 1998; (“Baldo-I”) and Baldo et al., “Very high-efficiency green organic light-emitting devices based on electrophosphorescence,” Appl. Phys. Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporated by reference in their entireties. Phosphorescence is described in more detail in U.S. Pat. No. 7,279,704 at cols. 5-6, which are incorporated by reference.

FIG. 1 shows an organic light emitting device 100. The figures are not necessarily drawn to scale. Device 100 may include a substrate 110, an anode 115, a hole injection layer 120, a hole transport layer 125, an electron blocking layer 130, an emissive layer 135, a hole blocking layer 140, an electron transport layer 145, an electron injection layer 150, a protective layer 155, and a cathode 160. Cathode 160 is a compound cathode having a first conductive layer 162 and a second conductive layer 164. Device 100 may be fabricated by depositing the layers described, in order. The properties and functions of these various layers, as well as example materials, are described in more detail in U.S. Pat. No. 7,279,704 at cols. 6-10, which are incorporated by reference.

More examples for each of these layers are available. For example, a flexible and transparent substrate-anode combination is disclosed in U.S. Pat. No. 5,844,363, which is incorporated by reference in its entirety. An example of a p-doped hole transport layer is m-MTDATA doped with F.sub.4-TCNQ at a molar ratio of 50:1, as disclosed in U.S. Patent Application Publication No. 2003/0230980, which is incorporated by reference in its entirety. Examples of emissive and host materials are disclosed in U.S. Pat. No. 6,303,238 to Thompson et al., which is incorporated by reference in its entirety. An example of an n-doped electron transport layer is BPhen doped with Li at a molar ratio of 1:1, as disclosed in U.S. Patent Application Publication No. 2003/0230980, which is incorporated by reference in its entirety. U.S. Pat. Nos. 5,703,436 and 5,707,745, which are incorporated by reference in their entireties, disclose examples of cathodes including compound cathodes having a thin layer of metal such as Mg:Ag with an overlying transparent, electrically-conductive, sputter-deposited ITO layer. The theory and use of blocking layers is described in more detail in U.S. Pat. No. 6,097,147 and U.S. Patent Application Publication No. 2003/0230980, which are incorporated by reference in their entireties. Examples of injection layers are provided in U.S. Patent Application Publication No. 2004/0174116, which is incorporated by reference in its entirety. A description of protective layers may be found in U.S. Patent Application Publication No. 2004/0174116, which is incorporated by reference in its entirety.

FIG. 2 shows an inverted OLED 200. The device includes a substrate 210, a cathode 215, an emissive layer 220, a hole transport layer 225, and an anode 230. Device 200 may be fabricated by depositing the layers described, in order. Because the most common OLED configuration has a cathode disposed over the anode, and device 200 has cathode 215 disposed under anode 230, device 200 may be referred to as an “inverted” OLED. Materials similar to those described with respect to device 100 may be used in the corresponding layers of device 200. FIG. 2 provides one example of how some layers may be omitted from the structure of device 100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided by way of non-limiting example, and it is understood that embodiments of the invention may be used in connection with a wide variety of other structures. The specific materials and structures described are exemplary in nature, and other materials and structures may be used. Functional OLEDs may be achieved by combining the various layers described in different ways, or layers may be omitted entirely, based on design, performance, and cost factors. Other layers not specifically described may also be included. Materials other than those specifically described may be used. Although many of the examples provided herein describe various layers as comprising a single material, it is understood that combinations of materials, such as a mixture of host and dopant, or more generally a mixture, may be used. Also, the layers may have various sublayers. The names given to the various layers herein are not intended to be strictly limiting. For example, in device 200, hole transport layer 225 transports holes and injects holes into emissive layer 220, and may be described as a hole transport layer or a hole injection layer. In one embodiment, an OLED may be described as having an “organic layer” disposed between a cathode and an anode. This organic layer may comprise a single layer, or may further comprise multiple layers of different organic materials as described, for example, with respect to FIGS. 1 and 2.

Structures and materials not specifically described may also be used, such as OLEDs comprised of polymeric materials (PLEDs) such as disclosed in U.S. Pat. No. 5,247,190 to Friend et al., which is incorporated by reference in its entirety. By way of further example, OLEDs having a single organic layer may be used. OLEDs may be stacked, for example as described in U.S. Pat. No. 5,707,745 to Forrest et al, which is incorporated by reference in its entirety. The OLED structure may deviate from the simple layered structure illustrated in FIGS. 1 and 2. For example, the substrate may include an angled reflective surface to improve out-coupling, such as a mesa structure as described in U.S. Pat. No. 6,091,195 to Forrest et al., and/or a pit structure as described in U.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated by reference in their entireties.

Unless otherwise specified, any of the layers of the various embodiments may be deposited by any suitable method. For the organic layers, preferred methods include thermal evaporation, ink-jet, such as described in U.S. Pat. Nos. 6,013,982 and 6,087,196, which are incorporated by reference in their entireties, organic vapor phase deposition (OVPD), such as described in U.S. Pat. No. 6,337,102 to Forrest et al., which is incorporated by reference in its entirety, and deposition by organic vapor jet printing (OVJP), such as described in U.S. patent application Ser. No. 10/233,470, which is incorporated by reference in its entirety. Other suitable deposition methods include spin coating and other solution based processes. Solution based processes are preferably carried out in nitrogen or an inert atmosphere. For the other layers, preferred methods include thermal evaporation. Preferred patterning methods include deposition through a mask, cold welding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819, which are incorporated by reference in their entireties, and patterning associated with some of the deposition methods such as ink-jet and OVJD. Other methods may also be used. The materials to be deposited may be modified to make them compatible with a particular deposition method. For example, substituents such as alkyl and aryl groups, branched or unbranched, and preferably containing at least 3 carbons, may be used in small molecules to enhance their ability to undergo solution processing. Substituents having 20 carbons or more may be used, and 3-20 carbons is a preferred range. Materials with asymmetric structures may have better solution processability than those having symmetric structures, because asymmetric materials may have a lower tendency to recrystallize. Dendrimer substituents may be used to enhance the ability of small molecules to undergo solution processing.

Devices fabricated in accordance with embodiments of the invention may be incorporated into a wide variety of consumer products, including flat panel displays, computer monitors, televisions, billboards, lights for interior or exterior illumination and/or signaling, heads up displays, fully transparent displays, flexible displays, laser printers, telephones, cell phones, personal digital assistants (PDAs), laptop computers, digital cameras, camcorders, viewfinders, micro-displays, vehicles, a large area wall, theater or stadium screen, or a sign. Various control mechanisms may be used to control devices fabricated in accordance with the present invention, including passive matrix and active matrix. Many of the devices are intended for use in a temperature range comfortable to humans, such as 18 degrees C. to 30 degrees C., and more preferably at room temperature (20-25 degrees C.).

The materials and structures described herein may have applications in devices other than OLEDs. For example, other optoelectronic devices such as organic solar cells and organic photodetectors may employ the materials and structures. More generally, organic devices, such as organic transistors, may employ the materials and structures.

The terms halo, halogen, alkyl, cycloalkyl, alkenyl, alkynyl, arylkyl, heterocyclic group, aryl, aromatic group, and heteroaryl are known to the art, and are defined in U.S. Pat. No. 7,279,704 at cols. 31-32, which are incorporated herein by reference.

There are reports of azaborine compounds in the literature (see, e.g., Kranz et al., Chem. Commun. 1247 (1992); Agou et al., Chem. Commun. 3204 (2007); Marwitz et al., Angew. Chem. Int. Ed. 48, 973 (2009); Pan et al., Organometallics 23, 5626 (2004); Pan et al., Organometallics 25, 197 (2006); Ashe et al., Organometallics 20, 5413 (2001); Ashe and Fang., Org. Lett. 2, 2089 (2000); Davies et al., J. Am. Chem. Soc. 89, 6294 (1967); Fiedler et al., Inorg. Chem. 35, 3039 (1996); Whire D. G., J. Am. Chem. Soc. 85, 3634 (1963); US2003/0157366; Blomberg et al., Tetrahedron 62, 10937 (2006); Fang et al., Organometallics 25, 513 (2006); Fang et al., Organometallics 27, 2892 (2008); Chen et al., Organometallics 23, 5088 (2004); Ashe et al., Organometallics 21, 4578 (2002); Abbey et al., Amer. Chem. Soc. 130, 7250 (2008)). However, the compounds containing a boron-nitrogen heterocycle provided herein have a novel structure. These organometallic compounds comprise a ligand containing a boron-nitrogen heterocycle. Such compounds may include ligands comprising a boron-nitrogen heterocycle and a carbocyclic ring, such as phenyl, wherein the ligand is coordinated to a metal. Compounds may also include ligands comprising a boron-nitrogen heterocycle and a N-containing ring, such as pyridine. In addition, these compounds may be advantageously used in OLEDs.

Inorganic/organic hybrids of benzene include 1,2-azaborine, 1,3-azaborine, and 1,4-azaborine. Table 1 below shows the structure of several exemplary boron-nitrogen heterocycles.

TABLE 1

It should be noted that each boron-nitrogen heterocycle provided can be drawn in various resonance structures. The resonance structures provided in Table 1 for 1,2-azaborine and 1,4-azaborine are exemplary resonance structures. Without being bound by theory, it is believed that compound containing 1,2-azaborine and 1,4-azaborine may be particularly beneficial. The resonance structures for these compounds show that conjugation may be disrupted by the presence of heteroatoms (i.e., nitrogen and/or boron) in the ring. The broken conjugation may provide high triplet energy for the compounds, which may be a beneficial property.

Using 1,2-azaborine, 1,3-azaborine, 1,4-azaborine, borazine or modifications of these organic/inorganic benzene to replace the regular aromatic moiety in the ligand used in a metal complex, such as Ir(2-phenylpyridine)₃, offers a new use for semi-aromatic, semi-conjugated compounds. The organic/inorganic benzene compounds provided herein include boron-nitrogen containing heterocycles with boron and nitrogen atoms in multiple positions within the ring including a ring having multiple boron atoms and multiple nitrogen atoms (i.e., 2N, 2B heterocycles). In particular, heterocycles containing two nitrogen atoms and two boron atoms (i.e., 2N, 2B) within the ring are believed to be a novel azaborine structure and may be particularly useful. In addition, the boron-nitrogen heterocycle may be further substituted by a variety of substituents.

In addition, the compounds including 1,2-azaborine, 1,3-azaborine, 1,4-azaborine may be advantageously used in organic electronics. 1,2-azaborine, 1,3-azaborine, 1,4-azaborine, borazine or modifications of these organic/inorganic benzene compounds can be used to replace the benzene ring found in many organic compounds. In particular, the semi-aromatic nature of azaborine may be useful to tune the electronic and photophysical properties of phenylpyridine-based compounds such as Ir(2-phenylpyridine)₃. Table 2 includes exemplary 2-azaborinepyridine complexes with iridium(III). For each of the compounds provided in Table 2, the calculated energy levels and the singlet and triplet energies (DFT, Gaussian/B3lyp/cep-31g) are also provided.

TABLE 2 HOMO LUMO Gap Dipole S1 Compound Structure (eV) (eV) (eV) (Debye) (nm) T1 (nm) Ir(ppy)₃

−4.93 −1.43 3.5 6.53 452 505 ppy

−6.23 −1.41 4.82 2.07 315 454 A

−5.07 −1.72 3.35 9.45 457 537 B

−4.7 −1.26 3.45 6.78 483 569 C

−5.17 −1.56 3.61 7.27 427 511 D

−5.46 −1.83 3.63 14.82 437 460 D−Ligand

−6.25 −1.56 −4.69 5.79 319 393 E

−5.47 −1.94 3.53 16.72 431 454 F

−4.53 −1.25 3.28 6.7 514 597 G

−4.06 −1.15 2.91 5.16 562 627 H

−5.46 −1.78 3.67 14.34 422 468 I

−4.73 −1.17 3.57 4.72 446 475 I−Ligand

−5.91 −1.46 4.44 3.65 344 470

From the calculations in Table 2, it can be seen that depending on the position of the boron atom and the nitrogen atom in the azaborine, a wide range of triplet energies (phosphorescence color) can be achieved. Most notably, Compounds D and E have phosphorescence in the deep blue region (≦460 nm) with very low LUMO levels (<1.8 eV). Such features may render the compounds more stable towards reduction which is believed to be desirable for long OLED device operation lifetime.

Another desirable feature is the triplet energy difference between the ligand and the corresponding metal complex. It is believed that a large triplet energy difference indicates a metal complex with a strong MLCT excited state character. It may also be viewed as a result of strong metal-organic bonding, in this case, Ir—N bond in the top ring and Ir—C/N/B bond in the lower ring. Strong metal-organic bonding is believed to be desirable to obtaining a stable complex for device operation. In Ir(ppy)₃, the most fundamental Ir tris C—N cyclometalating complex, the triplet energies of the complex and the ligand (ppy) are 505 nm (2.48 eV) and 454 nm (2.75 eV) respectively. The difference is 0.27 eV. Ir(ppy)₃ has a strong MLCT excited state character. In Compound D, the triplet energies of the complex D and the ligand (D-Ligand) are 460 nm (2.72 eV) and 393 nm (3.18 eV) respectively. The difference is 0.46 eV which is even higher than that of Ir(ppy)₃, suggesting Compound D has a very strong MLCT excited state character. In Compound I, a blue emitting Ir complex described in US 20080297033, the triplet energies of the complex and the ligand (I-Ligand) are 475 nm (2.63 eV) and 470 nm (2.66 eV) respectively. The difference is only 0.03 eV, suggesting a very weak MLCT excited state character. In fact, the emission profile of Compound I suggests a predominant ligand π to π* or LLCT transition.

As discussed above, the compounds may be further substituted thereby allowing additional tuning of the compound's properties, such as color and energy levels. In particular, Compound B and Compound F both contain a boron-nitrogen heterocycle ligand having 1,2-azaborine yet the two compounds have different properties which may be due to the methyl substitution of the 1,2-azaborine moiety in the ligand of Compound F.

Additionally, the orientation of the boron-nitrogen ligand with respect to the metal may also influence properties of the compound. In particular, Compound A and Compound B have the same ligand structure but differ in the coordination of the ligand to Ir and thus display different properties.

Moreover, the linkage between the boron-nitrogen containing heterocycle and the metal may also influence the compound's properties. In particular, Compound B and Compound C both contain 1,2-azaborine as the boron-nitrogen heterocycle moiety in the ligand. However, Compound B is coordinated to the Ir via a boron atom whereas Compound C is coordinated to the Ir via a nitrogen atom. The different heteroatomic linkage to the metal of the compound may provide altered electronic and photophysical properties.

Novel compounds are provided, the compounds comprising a ligand containing a boron-nitrogen heterocycle. These compounds may be particularly useful as phosphorescent emitters in OLEDs. The compounds contain a boron-nitrogen heterocycle in place of the aromatic moiety (e.g. phenyl) or heteroaromatic moiety typically present in a ligand. Phenylpyridine is a ligand commonly used in metal complexes. In particular, Ir(ppy) type complexes contain a phenylpyridine ligand comprising a pyridine ring and a phenyl ring. The compounds provided herein differ structurally from traditional compounds by the rings present in the ligands. The compounds herein may comprise, for example, a ligand containing a pyridine ring and a boron-nitrogen heterocycle.

B—N (or B′═N⁺) bonds and the C═C bonds are isoelectronic and isostructural. When coordinated to a metal, a boron-nitrogen heterocycle ligand has a charge such that the top ring (i.e., A ring) is a neutral donor and the bottom ring (i.e., B ring) is a monoanionic donor. Compounds comprising a boron-nitrogen heterocycle containing ligand are analogous and isoelectronic to the corresponding Ir(ppy) type complexes due to the similarities between the B—N bond and the C═C bond. Preferably, compounds comprising the novel ligand structure provided herein (i.e., ligand comprising an azaborine moiety and a carbocyclic or N-containing ring) may be isoelectronic and isostructural to corresponding Ir(ppy) type complexes. In particular, compounds having B—N bonds have equivalent valence electrons and equivalent structure (i.e., atom connectivity) as compared to Ir(ppy) type compounds.

In addition, metal complexes comprising a boron-nitrogen heterocycle containing ligand may have stronger bonds than Ir(ppy) type complex.

Ligands containing a boron-nitrogen heterocycle provide a novel ring system useful for tuning the photophysical (e.g., color) and electrochemical (e.g., energy levels) properties of metal compounds. As such, compounds comprising a boron-nitrogen heterocycle containing ligand may be useful in red, green, and blue devices. In particular, boron-nitrogen heterocycles may be especially useful in blue devices. Specifically, ligands consisting of 1,2-azaborine or 1,4-azaborine may be especially desirable for use in blue devices. A disruption in conjugation between the two rings of the ligand (i.e., the A ring and the B ring) may provide high triplet energy. The 1,2-azaborine containing ligand may have some conjugation between the two rings in the ligand (i.e., A and B), but the conjugation is less than that of a common ligand such as phenylpyridine. The 1,4-azaborine ligand is believed to have even less conjugation between the two rings. In particular, Compound D (Table 2) is a 1,4-azaborine containing compound and has little conjugation between the pyridine and the azaborine. Thus, the 1,4-azaborine ligand may be the most preferred organic/inorganic benzene hybrid provided herein for use in blue devices because it may provide the highest triplet energy.

Additionally, the atomic connection between the boron-nitrogen containing ring (i.e., B ring) and the other ring (i.e., A ring) of the ligand may contribute to triplet energy. In particular, a heteroatomic linkage between the two rings of the ligand may be especially beneficial. The presence of boron in the B ring at the 1 position provides a heteroatomic linkage between the two rings in the ligand. The presence of nitrogen at the 1 position in a ring containing boron may also provide the heteroatomic linkage. The compounds having a heteroatomic linkage may be especially good compounds for phosphorescent blue devices.

While the use of compounds containing a boron-nitrogen heterocycle as blue emitters is a preferred use, such compounds may be used for other purposes as well, including the emission of other colors, as well as non-emissive uses.

The compounds provided herein comprise a ligand L having the structure:

A and B may represent a 5-membered or 6-membered carbocycle or heterocycle. X₁ is selected from the group consisting of carbon and nitrogen. Preferably, X₁ is nitrogen.

More preferably, A is:

In certain aspects, it may be preferred that R₁ is hydrogen. X₂ and X₃ are independently selected from the group consisting of carbon, nitrogen, and boron. B is a boron-nitrogen heterocycle. In one aspect, A may be selected from the group consisting of:

X is selected from the group consisting of S, NZ, O, Se, BZ, CZZ′, and C═O. Z and Z′ are independently selected from the group consisting of hydrogen, alkyl, and aryl.

Additionally, A may be selected from the group consisting of:

X is selected from the group consisting of S, NZ, O, Se, BZ, CZZ′, and C═O. Z and Z′ are independently selected from the group consisting of hydrogen, alkyl, and aryl.

R₁ and R₂ represent mono, di, tri, or tetra substitutions. R₁ and R₂ are independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl.

The ligand L is coordinated to a metal M having an atomic number greater than 40. Preferably, the metal M is Ir.

In one aspect, compounds are provided wherein the compound includes a ligand L having the structure:

X₄, X₅, X₆, and X₇ are independently selected from the group consisting of carbon, nitrogen, and boron.

In another aspect, compounds are provided wherein the compound includes a ligand L having the structure:

X₄, X₅, and X₆ are independently selected from the group consisting of carbon, nitrogen, and boron.

Specific examples of the ligand L are provided, and include:

R₃, R₄, R₅, and R₆ are independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl.

Preferably, the ligand L is selected from the group consisting of:

More preferably, the ligand L is:

In one aspect, R₂ is hydrogen. Preferably, R₃ is selected from the group consisting of alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl. In particular, compounds containing minimal NH and BH groups on the ring may be preferred. Without being bound by theory, it is believed that NH and BH groups are very reactive. Therefore, compounds lacking or containing a minimal number of these groups on the ring may be more stable and thus preferred.

In one aspect, the ligand L is included in a homoleptic compound. In another aspect, the ligand L is included in a heteroleptic compound.

In a particular aspect, compounds having the formula M^(n)(L)_(a)(L′)_(b)(L″)_(c) are provided. n is the oxidation state of the metal M. The ligands may be combined in a variety of ways. In one aspect, the ligands may all be the same structure. In another aspect, the can all have a different structure. In yet another aspect, 2 of the ligands may be the same and one of the ligands may be different. In particular, the ligands L′ and L″ can be phenylpyridine or phenylpyridine derivatives (i.e., FORMULA IV), boron-nitrogen containing heterocycles (i.e., FORMULA I), or other ligands.

In one aspect, n is 3. a is 1, 2, or 3. b is 0, 1, or 2. c is 0, 1, or 2. a+b+c is n. In one aspect, preferably a is 1. In another aspect, preferably a is 2. In compounds having the formula M^(n)(L)_(a)(L′)_(b)(L″)_(c) wherein a is 1 or 2, the compound is a heteroleptic compound. In yet another aspect, preferably a is 3. In compounds having the formula M^(n)(L)_(a)(L′)_(b)(L″)_(c) wherein a is 3, the compound is a homoleptic compound.

L′ and L″ are independently selected from the group consisting of:

R′, R″ and R′″ are independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl. X is selected from the group consisting of S, NZ, O, Se, BZ, CZZ′, and C═O. Z and Z′ are independently selected from the group consisting of hydrogen, alkyl, and aryl.

Heteroleptic compounds comprising a phenylpyridine ligand, optionally substituted with a wide variety of substituents and/or heteroatoms, may be particularly useful. Thus, heteroleptic compounds having L′ and/or L″ which comprise a ligand structure having FORMULA IV may be beneficial. Preferably, L′ and L″ are independently selected from the group consisting of:

Ligands having the structure of Compound 68-Compound 83 may be further substituted, as described above. For some applications, compounds wherein substituents R′, R″, and R′″ are hydrogen may be preferred.

The heteroleptic compounds and homoleptic compounds provided herein may be used in red, green and blue devices. In particular, the ligands L′ and L″ of heteroleptic compounds may influence the emissive properties of the compound and therefore the ligands L′ and L″ which are included in the compound may differ depending on the device in which the compound may be used (i.e., red, green, or blue). In particular, a heteroleptic compound having phenylimidazole as the ligand L′ and/or L″ may be especially useful in a blue device. A heteroleptic compound having phenylpyridine as the ligand L′ and/or L″ may be especially useful in a green device. A heteroleptic compound having phenylquinoline as the ligand L′ and/or L″ may be especially useful in a red device.

In one aspect, the compound is selected from the group consisting of:

More particularly, the compound may be selected from the group consisting of:

In one particular aspect, the compound is:

In another particular aspect, the compound is:

Additional compounds comprising a boron-nitrogen heterocycle are provided, such compounds having the formula:

A and B may represent a 5-membered or 6-membered carbocycle or heterocycle. X₁ is N or NR. X₂ and X₃ are independently selected from the group consisting of carbon, nitrogen, and boron. B is a boron-nitrogen heterocycle. R, R₁ and R₂ represent mono, di, tri, or tetra substitutions. R, R₁ and R₂ are independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl.

Additionally, an organic light emitting device is provided. The device comprises an anode, a cathode, and an organic layer disposed between the anode and the cathode. The organic layer further comprises a compound comprising a ligand L having the structure FORMULA I, as discussed above.

Selections for the rings, metal, substituents, and ligands described as preferred for compounds comprising a ligand L having FORMULA I are also preferred for use in a device that contains a compound comprising a ligand L having FORMULA I. These selections include those for rings A and B, the metal M, the substituents X₁-X₃, the substituents R₁-R₆, R′, R″, R′″, and the combination and/or incorporation of ligands L′ and L″ into an organometallic complex.

Specific examples of the ligand having FORMULA I are provided, and include a ligand L selected from the group consisting of Compound 1-Compound 67. Preferably, the ligand L is selected from the group consisting of Compound 1, Compound 6, Compound 7, Compound 12, Compound 25, and Compound 28. More preferably, the ligand L is Compound 25. In particular, the substituent R₂ of Compound 25 may be hydrogen.

In a particular aspect, devices are provided wherein the device containing a compound having the formula M^(n)(L)_(a)(L′)_(b)(L″)_(c). n is the oxidation state of the metal M. a is 1, 2, or 3. b is 0, 1, or 2. c is 0, 1, or 2. a+b+c is n. In one aspect, preferably a is 1. In another aspect, preferably a is 2. In yet another aspect, preferably a is 3. L′ and L″ are independently selected from the group consisting of FORMULA I, FORMULA IV, FORMULA V, FORMULA VI, FORMULA VII, FORMULA VIII, FORMULA IX, and FORMULA X, as discussed above. In particular, the ligands L′ and L″ are independently selected from the group consisting of Compound 68-Compound 83.

Devices containing particular boron-nitrogen heterocyclic compounds are also provided. Examples of the particular compounds include compounds selected from the group consisting of Compound 84G-Compound 167G. Additionally, devices are provided which contain a compound comprising a specific boron-nitrogen containing heterocycle, the compound is selected from the group consisting of Compound 84-Compound 167.

Devices having certain structures are also provided. In particular, devices are provided wherein the organic layer is an emissive layer and the compound comprising the ligand L having FORMULA I is an emitting dopant. Preferably, the organic layer further comprises a host containing a benzene, carbazole, dibenzofuran, dibenzothiophene, dibenzoselenophene, naphthalene, triphenylene, or biphenyl moiety.

A consumer product comprising a device is also provided. The device comprises an anode, a cathode, and an organic layer, disposed between the anode and the cathode. The organic layer comprises a compound comprising a ligand L having the formula of FORMULA I,

Selections for the rings, metal, substituents, and ligands described as preferred for compounds comprising a ligand L having FORMULA I are also preferred for use in a consumer product comprising a device that contains a compound comprising a ligand L having FORMULA I. These selections include those for rings A and B, the metal M, the substituents X₁-X₃, the substituents R₁-R₆, R′, R″, R′″, and the combination and/or incorporation of ligands L′ and L″ into an organometallic complex.

The materials described herein as useful for a particular layer in an organic light emitting device may be used in combination with a wide variety of other materials present in the device. For example, emissive dopants disclosed herein may be used in conjunction with a wide variety of hosts, transport layers, blocking layers, injection layers, electrodes and other layers that may be present. The materials described or referred to below are non-limiting examples of materials that may be useful in combination with the compounds disclosed herein, and one of skill in the art can readily consult the literature to identify other materials that may be useful in combination.

In addition to and/or in combination with the materials disclosed herein, many hole injection materials, hole transporting materials, host materials, dopant materials, exiton/hole blocking layer materials, electron transporting and electron injecting materials may be used in an OLED. Non-limiting examples of the materials that may be used in an OLED in combination with materials disclosed herein are listed in Table 3 below. Table 3 lists non-limiting classes of materials, non-limiting examples of compounds for each class, and references that disclose the materials.

HIL/HTL:

A hole injecting/transporting material to be used in the present invention is not particularly limited, and any compound may be used as long as the compound is typically used as a hole injecting/transporting material. Examples of the material include, but not limit to: a phthalocyanine or porphyrin derivative; an aromatic amine derivative; an indolocarbazole derivative; a polymer containing fluorohydrocarbon; a polymer with conductivity dopants; a conducting polymer, such as PEDOT/PSS; a self-assembly monomer derived from compounds such as phosphonic acid and silane derivatives; a metal oxide derivative, such as MoO_(x); a p-type semiconducting organic compound, such as 1,4,5,8,9,12-Hexaazatriphenylenehexacarbonitrile; a metal complex, and a cross-linkable compounds.

Examples of aromatic amine derivatives used in HIL or HTL include, but not limit to the following general structures:

Each of Ar¹ to Ar⁹ is selected from the group consisting aromatic hydrocarbon cyclic compounds such as benzene, biphenyl, triphenyl, triphenylene, naphthalene, anthracene, phenalene, phenanthrene, fluorene, pyrene, chrysene, perylene, azulene; group consisting aromatic heterocyclic compounds such as dibenzothiophene, dibenzofuran, dibenzoselenophene, furan, thiophene, benzofuran, benzothiophene, benzoselenophene, carbazole, indolocarbazole, pyridylindole, pyrrolodipyridine, pyrazole, imidazole, triazole, oxazole, thiazole, oxadiazole, oxatriazole, dioxazole, thiadiazole, pyridine, pyridazine, pyrimidine, pyrazine, triazine, oxazine, oxathiazine, oxadiazine, indole, benzimidazole, indazole, indoxazine, benzoxazole, benzisoxazole, benzothiazole, quinoline, isoquinoline, cinnoline, quinazoline, quinoxaline, naphthyridine, phthalazine, pteridine, xanthene, acridine, phenazine, phenothiazine, phenoxazine, benzofuropyridine, furodipyridine, benzothienopyridine, thienodipyridine, benzoselenophenopyridine, and selenophenodipyridine; and group consisting 2 to 10 cyclic structural units which are groups of the same type or different types selected from the aromatic hydrocarbon cyclic group and the aromatic heterocyclic group and are bonded to each other directly or via at least one of oxygen atom, nitrogen atom, sulfur atom, silicon atom, phosphorus atom, boron atom, chain structural unit and the aliphatic cyclic group. Wherein each Ar is further substituted by a substituent selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylalkyl, heteroalkyl, aryl and heteroaryl.

In one aspect, Ar¹ to Ar⁹ is independently selected from the group consisting of:

k is an integer from 1 to 20; X¹ to X⁸ is CH or N; Ar¹ has the same group defined above.

Examples of metal complexes used in HIL or HTL include, but not limit to the following general formula:

M is a metal, having an atomic weight greater than 40; (Y¹-Y²) is a bindentate ligand, Y¹ and Y² are independently selected from C, N, O, P, and S; L is an ancillary ligand; m is an integer value from 1 to the maximum number of ligands that may be attached to the metal; and m+n is the maximum number of ligands that may be attached to the metal.

In one aspect, (Y¹-Y²) is a 2-phenylpyridine derivative.

In another aspect, (Y¹-Y²) is a carbene ligand.

In another aspect, M is selected from Ir, Pt, Os, and Zn.

In a further aspect, the metal complex has a smallest oxidation potential in solution vs. Fc⁺/Fc couple less than about 0.6 V.

Host:

The light emitting layer of the organic EL device of the present invention preferably contains at least a metal complex as light emitting material, and may contain a host material using the metal complex as a dopant material. Examples of the host material are not particularly limited, and any metal complexes or organic compounds may be used as long as the triplet energy of the host is larger than that of the dopant.

Examples of metal complexes used as host are preferred to have the following general formula:

M is a metal; (Y³-Y⁴) is a bindentate ligand, Y³ and Y⁴ are independently selected from C, N, O, P, and S; L is an ancillary ligand; m is an integer value from 1 to the maximum number of ligands that may be attached to the metal; and m+n is the maximum number of ligands that may be attached to the metal.

In one aspect, the metal complexes are:

(O—N) is a bidentate ligand, having metal coordinated to atoms O and N.

In another aspect, M is selected from Ir and Pt.

In a further aspect, (Y³-Y⁴) is a carbene ligand.

Examples of organic compounds used as host are selected from the group consisting aromatic hydrocarbon cyclic compounds such as benzene, biphenyl, triphenyl, triphenylene, naphthalene, anthracene, phenalene, phenanthrene, fluorene, pyrene, chrysene, perylene, azulene; group consisting aromatic heterocyclic compounds such as dibenzothiophene, dibenzofuran, dibenzoselenophene, furan, thiophene, benzofuran, benzothiophene, benzoselenophene, carbazole, indolocarbazole, pyridylindole, pyrrolodipyridine, pyrazole, imidazole, triazole, oxazole, thiazole, oxadiazole, oxatriazole, dioxazole, thiadiazole, pyridine, pyridazine, pyrimidine, pyrazine, triazine, oxazine, oxathiazine, oxadiazine, indole, benzimidazole, indazole, indoxazine, benzoxazole, benzisoxazole, benzothiazole, quinoline, isoquinoline, cinnoline, quinazoline, quinoxaline, naphthyridine, phthalazine, pteridine, xanthene, acridine, phenazine, phenothiazine, phenoxazine, benzofuropyridine, furodipyridine, benzothienopyridine, thienodipyridine, benzoselenophenopyridine, and selenophenodipyridine; and group consisting 2 to 10 cyclic structural units which are groups of the same type or different types selected from the aromatic hydrocarbon cyclic group and the aromatic heterocyclic group and are bonded to each other directly or via at least one of oxygen atom, nitrogen atom, sulfur atom, silicon atom, phosphorus atom, boron atom, chain structural unit and the aliphatic cyclic group. Wherein each group is further substituted by a substituent selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylalkyl, heteroalkyl, aryl and heteroaryl.

In one aspect, host compound contains at least one of the following groups in the molecule:

R¹ to R⁷ is independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylalkyl, heteroalkyl, aryl and heteroaryl, when it is aryl or heteroaryl, it has the similar definition as Ar's mentioned above;

k is an integer from 0 to 20;

X¹ to X⁸ is selected from CH or N.

HBL:

A hole blocking layer (HBL) may be used to reduce the number of holes and/or excitons that leave the emissive layer. The presence of such a blocking layer in a device may result in substantially higher efficiencies as compared to a similar device lacking a blocking layer. Also, a blocking layer may be used to confine emission to a desired region of an OLED.

In one aspect, compound used in HBL contains the same molecule used as host described above.

In another aspect, compound used in HBL contains at least one of the following groups in the molecule:

k is an integer from 0 to 20; L is an ancillary ligand, m is an integer from 1 to 3.

ETL:

Electron transport layer (ETL) may include a material capable of transporting electrons. Electron transport layer may be intrinsic (undoped), or doped. Doping may be used to enhance conductivity. Examples of the ETL material are not particularly limited, and any metal complexes or organic compounds may be used as long as they are typically used to transport electrons.

In one aspect, compound used in ETL contains at least one of the following groups in the molecule:

R¹ is selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylalkyl, heteroalkyl, aryl and heteroaryl, when it is aryl or heteroaryl, it has the similar definition as Ar's mentioned above;

Ar¹ to Ar³ has the similar definition as Ar's mentioned above;

k is an integer from 0 to 20;

X¹ to X⁸ is selected from CH or N.

In another aspect, the metal complexes used in ETL contains, but not limit to the following general formula:

(O—N) or (N—N) is a bidentate ligand, having metal coordinated to atoms O, N or N,N; L is an ancillary ligand; m is an integer value from 1 to the maximum number of ligands that may be attached to the metal.

EXPERIMENTAL Compound Examples Example 1 Synthesis of Ligand in Compound 88

Synthesis of N-(pyridin-2-yl)formamide

To a solution of sodium methoxide (176 mL, 25 wt %, 0.81 mol) in 500 mL of methanol was added 2-aminopyridine (30.0 g, 0.32 mol) and the resulting solution was stirred at 40° C. for 30 minutes. Ethyl formate (220 mL, 2.76 mol) was then added dropwise and the reaction mixture was stirred overnight at 40° C. After cooling to room temperature, 250 mL of H₂O and 250 mL of CH₂Cl₂ were added and conc. HCl (˜40 mL) was added dropwise until pH ˜5. Most of the methanol was evaporated and the resulting mixture was extracted with 3×150 mL of CH₂Cl₂. The combined organic portions were washed with 250 mL of H₂O and 150 mL of brine and dried over Na₂SO₄. After evaporation of the solvent, 20.2 g of crude solid was obtained and distilled on a Kugelrohr at an oven temperature of 140° C. to yield 18.98 g of a white solid as the product.

Synthesis of Acetic Formic Anhydride

Acetyl chloride (41 mL, 0.577 mol) was added rapidly through an addition funnel to a mixture of sodium formate (59 g, 0.86 mol) in 50 mL of ether in an ice/salt bath. The reaction mixture was stirred overnight and the resulting acetic formic anhydride ethereal solution was used without further purification.

Synthesis of N-formyl-N-(pyridin-2-yl)formamide

To a suspension of sodium hydride (10.8 g, 60 wt %, 0.27 mol) in 50 mL of THF was added dropwise a solution of N-(pyridine-2-yl)formamide (22.4 g, 0.18 mol) in 250 mL of THF. When the addition was complete, the reaction was stirred at room temperature for 30 minutes and then cooled in an ice/salt bath. The acetic formic anhydride ethereal solution from the previous step was added in one portion. The resulting mixture was stirred in the ice bath for 30 minutes and then overnight at room temperature. The mixture was poured into 1 L of ice/water and extracted with 3×250 mL of CH₂Cl₂. The combined organic extracts were washed with 2×250 mL of H₂O, dried and evaporated to give 33.2 g of solid. The crude product was column chromatographed (silica gel) eluting with 100% CH₂Cl₂ followed by 90:10 CH₂Cl₂:EtOAc. 14.2 g of product was isolated as a white solid.

Synthesis of N,N-bis(2,2-dibromovinyl)pyridin-2-amine

In a 2 L, 3-neck flask with magnetic stirrer, thermocouple, condenser, nitrogen inlet and addition funnel triphenylphosphine (82.5 g, 0.31 mol) and tetrabromomethane (103.5 g, 0.31 mol) was dissolved in 750 mL of CH₂Cl₂. Zinc (20.27 g, 0.31 mol) was added giving an orange mixture. The reaction mixture was cooled to ˜10° C. in an ice bath and stirred for 10 minutes. A solution of N-formyl-N-(pyridine-2-yl)formamide (5.9 g, 0.04 mol) in 100 mL of CH₂Cl₂ was added in a slow stream through the addition funnel. Pyridine (32.7 mL, 0.41 mol) was then added through the addition funnel. The reaction temperature rises from 10° C. to 24° C. and turns dark brown in color. The reaction mixture was stirred overnight, slowly warming to room temperature, and filtered through celite. The filtrate was washed with 2×250 mL of water and 250 mL of brine, dried and evaporated leaving ˜200 g of a black gummy solid. The crude material was chromatographed on a flash silica column eluted with 75:25 hexane:CH₂Cl₂ followed by 50:50 hexane:CH₂Cl₂ and finally 25:75 hexane:CH₂Cl₂, yielding 14.9 g of product.

Synthesis of N,N-bis((Z)-2-bromovinyl)pyridin-2-amine

An oven-dried 500 mL, 3-neck flask with magnetic stirrer, addition funnel, thermocouple and nitrogen inlet was charged with anhydrous zinc chloride (15.0 g, 0.11 mol) and cooled in an ice/salt bath. 100 mL of THF was then added dropwise. Butyllithium (134 mL, 2.5 M in hexanes, 0.34 mol) was then added dropwise such that the temperature was maintained below 0° C., giving a milky white solution that was stirred at 0° C. for 30 minutes. The ice/salt bath was replaced with a dry ice/acetone bath and the reaction cooled to −75° C. N,N-bis(2,2-dibromovinyl)pyridin-2-amine (5.0 g, 0.011 mol) in 50 mL of THF was added dropwise and the reaction stirred at −75° C. for 20 h. While still at −75° C., 100 mL of 10% acetic acid in THF was added dropwise and the cold solution poured into 150 mL of 1 N HCl. The reaction mixture was extracted with 3×100 mL of ether and the combined organics were washed with 2×100 mL of saturated NaHCO₃, dried and evaporated leaving 3.0 g of an colorless liquid as the product.

Synthesis of Ligand in Compound 88

^(t)BuLi (4.9 mL, 1.7 M in pentane, 8.3 mmol) was added dropwise to a solution of N,N-bis((Z)-2-bromovinyl)pyridin-2-amine (500 mg, 1.650 mmol) in 50 mL of Et₂O at −70° C. A precipitate was formed and the solution turned slightly darker to a pale orange mixture. The reaction mixture was stirred for 3 h at −70° C. during which time the precipitate went into solution. Dimethyl (2,6-dimethylphenyl)boronate (322 mg, 2.14 mmol) in 10 mL of ether was then added dropwise to the reaction solution, which was then allowed to warm slowly to room temperature. The reaction was filtered through a plug of Celite and concentrated to give 200 mg of a solid. GC/MS confirmed product formation.

Example 2 Synthesis of Ligand in Compound 87

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Synthesis of 2-vinylaniline

A mixture of 2-(2-aminophenyl)ethanol (5 g, 36.4 mmol) and KOH (2.0 g, 36.4 mmol) was heated to 200° C. for 4 h. After cooling, the mixture was distilled under vacuum to yield 2.1 g of product.

Synthesis of 2-chloro-1,2-dihydrobenzo[e][1,2]azaborine

BCl₃ (67.8 mL, 1.0 M in hexanes) was added to a solution if 2-vinylaniline (5.3 g, 44.3 mmol) in 40 mL of anhydrous toluene at 0° C. The mixture was stirred at 0° C. for 1 h and then refluxed for 2 h. The mixture was then evaporated to dryness and used immediately for next step.

Synthesis of Ligand in Compound 87

2-pyridylmagnesium bromide was generated by reacting isopropylmagnesium bromide (9.2 mL, 2.0 M in THF, 18.4 mmol) with 2-bromopyridine (2.9 g, 18.4 mmol) at room temperature for 2 h. This 2-pyridylmagnesium solution was then added to 2-chloro-1,2-dihydrobenzo[e][1,2]azaborine (1.0 g, 6.12 mmol) in anhydrous THF (30 mL) at −78° C. The mixture was stirred at −78° C. for 2 h and warm to room temperature. The mixture was poured into water and extracted with ethyl acetate. The organic extracts were dried and concentrated. The residue was column chromatographed (30% EtOAc in hexanes) to yield 0.5 g of product.

Example 3 Synthesis of Ligand in Compound 86

2-chloro-1-ethyl-1,2-azaborine (0.14 g, 1.0 mmol) in THF (5 mL) was add to a mixture of 2-(tributylstannyl)pyridine (0.45 g, 1.1 mmol) in THF (20 mL). The mixture was stirred at room temperature for 72 h. The reaction mixture was then diluted with ethyl acetate (50 mL) and washed with brine (50 mL). The organic layer was dried over sodium sulfate and evaporated in vacuo. GCMS and NMR confirmed product formation.

It is understood that the various embodiments described herein are by way of example only, and are not intended to limit the scope of the invention. For example, many of the materials and structures described herein may be substituted with other materials and structures without deviating from the spirit of the invention. The present invention as claimed may therefore includes variations from the particular examples and preferred embodiments described herein, as will be apparent to one of skill in the art. It is understood that various theories as to why the invention works are not intended to be limiting. 

1. A compound comprising a ligand L having the structure:

wherein A and B may represent a 5-membered or 6-membered carbocycle or heterocycle; wherein X₁ is selected from the group consisting of carbon and nitrogen; wherein X₂ and X₃ are independently selected from the group consisting of carbon, nitrogen, and boron; wherein B is a boron-nitrogen heterocycle; wherein R₁ and R₂ represent mono, di, tri, or tetra substitutions; wherein R₁ and R₂ are independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl; and wherein the ligand L is coordinated to a metal M having an atomic number greater than
 40. 2. The compound of claim 1, wherein A is:


3. The compound of claim 1, wherein the metal M is Ir.
 4. The compound of claim 1, wherein the ligand L has the structure:

wherein X₄, X₅, X₆, and X₇ are independently selected from the group consisting of carbon, nitrogen, and boron.
 5. The compound of claim 1, wherein the ligand L has the structure:

wherein X₄, X₅, and X₆ are independently selected from the group consisting of carbon, nitrogen, and boron.
 6. The compound of claim 1, wherein the ligand L is selected from the group consisting of:

wherein R₃, R₄, R₅, and R₆ are independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl.
 7. The compound of claim 1, wherein the compound has the formula M^(n)(L)_(a)(L′)_(b)(L″)_(c): wherein n is the oxidation state of the metal M; wherein a is 1, 2, or 3; wherein b is 0, 1, or 2; wherein c is 0, 1, or 2; wherein a+b+c is n; and wherein L′ and L″ are independently selected from the group consisting of:

wherein R′, R″, and R′″ are independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, aryl, and heteroaryl; wherein X is selected from the group consisting of S, NZ, O, Se, BZ, CZZ′, and C═O; and wherein Z and Z′ are independently selected from the group consisting of hydrogen, alkyl, and aryl.
 8. The compound of claim 7, wherein L′ and L″ are independently selected from the group consisting of:


9. The compound of claim 1, wherein the compound is selected from the group consisting of:


10. The compound of claim 9, wherein the compound is selected from the group consisting of:


11. An organic light emitting device comprising: an anode; a cathode; and an organic layer, disposed between the anode and the cathode, wherein the organic layer comprises a compound comprising a ligand L having the structure:

wherein A and B may represent a 5-membered or 6-membered carbocycle or heterocycle; wherein X₁ is selected from the group consisting of carbon and nitrogen; wherein X₂ and X₃ are independently selected from the group consisting of carbon, nitrogen, and boron; wherein B is a boron-nitrogen heterocycle; wherein R₁ and R₂ represent mono, di, tri, or tetra substitutions; wherein R₁ and R₂ are independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl; and wherein the ligand L is coordinated to a metal M having an atomic number greater than
 40. 12. The device of claim 11, wherein A is:


13. The device of claim 11, wherein the metal M is Ir.
 14. The device of claim 11, wherein the ligand L has the structure:

wherein X₄, X₅, X₆, and X₇ are independently selected from the group consisting of carbon, nitrogen, and boron.
 15. The device of claim 11, wherein the ligand L has the structure:

wherein X₄, X₅, and X₆ are independently selected from the group consisting of carbon, nitrogen, and boron.
 16. The device of claim 11, wherein the ligand L is selected from the group consisting of:

wherein R₃, R₄, R₅, and R₆ are independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl.
 17. The device of claim 11, wherein the compound has the formula M^(n)(L)_(a)(L′)_(b)(L″)_(c): wherein n is the oxidation state of the metal M; wherein a is 1, 2, or 3; wherein b is 0, 1, or 2; wherein c is 0, 1, or 2; wherein a+b+c is n; and wherein L′ and L″ are independently selected from the group consisting of:

wherein R′, R″, and R′″ is selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, aryl, and heteroaryl; wherein X is selected from the group consisting of S, NZ, O, Se, BZ, CZZ′, and C═O; and wherein Z and Z′ are independently selected from the group consisting of hydrogen, alkyl, and aryl.
 18. The device of claim 17, wherein the compound is selected from the group consisting of:


19. The device of claim 11, wherein the organic layer is an emissive layer and the compound comprising the ligand L having FORMULA I is an emitting dopant; and the organic layer further comprises a host containing a benzene, carbazole, dibenzofuran, dibenzothiophene, dibenzoselenophene, naphthalene, triphenylene, or biphenyl moiety.
 20. A consumer product comprising a device, the device further comprising: an anode; a cathode; and an organic layer, disposed between the anode and the cathode, wherein the organic layer comprises a compound comprising a ligand L having the formula:

wherein A and B may represent a 5-membered or 6-membered carbocycle or heterocycle; wherein X₁ is selected from the group consisting of carbon and nitrogen; wherein X₂ and X₃ are independently selected from the group consisting of carbon, nitrogen, and boron; wherein B is a boron-nitrogen heterocycle; wherein R₁ and R₂ represent mono, di, tri, or tetra substitutions; wherein R₁ and R₂ are independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl; and wherein the ligand L is coordinated to a metal M having an atomic number greater than
 40. 21. A compound having the formula:

wherein A and B may represent a 5-membered or 6-membered carbocycle or heterocycle; wherein X₁ is N or NR; wherein X₂ and X₃ are independently selected from the group consisting of carbon, nitrogen, and boron; wherein B is a boron-nitrogen heterocycle; wherein R, R₁ and R₂ represent mono, di, tri, or tetra substitutions; wherein R, R₁ and R₂ are independently selected from the group consisting of hydrogen, alkyl, alkoxy, amino, alkenyl, alkynyl, arylkyl, aryl, and heteroaryl. 